The production of wood chips for the paper and pulp industry is generally implemented using disc chippers, in which the wood is chipped by means of blades mounted in a radial, or nearly radial arrangement on a disc. As the disc rotates, the blades strike a log being chipped, the opposite side of which leans against a counter blade. The counter blade is located at the delivery end of a feed funnel, on the opposite side thereof relative to the direction of motion of the blade. The wood may be fed into the disc chipper either vertically or horizontally, and the chipper is accordingly called a horizontal-feed or a vertical-feed chipper. There are corresponding differences in the positions of the disc and blades in these different chipper types, but in principle they are essentially identical in operation.
In a disc chipper, the parts subject to wear are primarily the chipping blade and the counter blade, which receives the log being chipped. For the chips produced to be suitable for use as fibrous material, the counter blade must meet certain requirements regarding its condition. The cutting edge of the counter blade (i.e. the arris against which the log bears during chipping) must not have an excessive radius of curvature, otherwise the wood fibres will be broken and the result is low-quality chips containing sawdust.
In the known prior art, the counter blade consists of a relatively large and bulky body that requires a large amount of material and is difficult to handle during installation and maintenance. The wearing part of the counter blade is provided with a hard-metal coating which needs periodic renewal. At present, there are also counter blades of a lighter construction which are free of the above-mentioned drawbacks. However, there is still the problem that different parts of the cutting surface of the counter blade are unevenly worn due to the uneven distribution of logs. For this reason, the counter blade cannot be utilized in full.
To achieve an optimal chipping efficiency, the counter blade in a vertical-feed chipper consists of a horizontal part and another part placed in a direction slightly differing from the horizontal, so that the blade parts are at an angle slightly diverging from 180.degree. relative to each other. In a blade construction like this, the horizontal blade area close to the junction of the parts is subject to the hardest wear, due to the manner in which the logs are directed in the feed funnel, and so the wear in this area determines the need for blade replacement. Therefore, when this small area of the whole effective surface of the blade has worn out, the whole blade must be replaced. If the counter blade has a hard-metal coating, renewing the coating means re-coating the whole blade.